1. Field of the Invention
This invention relates generally to accelerometers and more particularly to a servo accelerometer.
2. Description of the Prior Art
The outline of a prior art accelerometer which was previously proposed by the present applicant (disclosed in Japanese patent application No. 188924/1981) will be described with reference to FIGS. 1 and 2.
FIG. 1 is a cross-sectional view of such previously proposed accelerometer and FIG. 2 is a partially cut-out plan cross-sectional view thereof.
In FIGS. 1 and 2, reference numeral 1 generally designates a pendulum of rod-shape for detecting acceleration and which is supported by a flexible joint, or hinge 3 having a thin portion 2, namely, a flexible portion. This pendulum 1 can rotate around the thin portion 2 of the hinge 3 only in the X--X direction which is in parallel to the sheet of drawing of FIG. 1. One end of the hinge 3 is positively fixed to a hinge mounting base portion 5 of a first base table or frame 4 through a hinge support plate 6 by, for example, a screw and so on. A pair of torquer coils 7 and 7' of cylindrical shape are attached to the pendulum 1 in the direction perpendicular to the rotation axis or O--O axis of the thin portion 2 of the hinge 3. A disk-shaped permanent magnet 8 and a circular columnar-shaped pole piece 9 are respectively attached to the first frame 4 by, for example, a bonding agent. One torquer coil 7 is within first frame 4. Since the first frame 4 is made of soft iron material of electromagnetic property, this first frame 4 forms a magnetic circuit with the permanent magnet 8 between the first frame 4 and the pole piece 9 and hence serves as a yoke (return path). The permanent magnet 8, the pole piece 9 and the first frame 4 constitute one torquer together with the torquer coil 7. Further, to the first frame 4 there is attached a displacement detecting device which consists of two light receiving elements 10 and 10' and a light emission element 11 so as to detect the displacement of the pendulum 1.
On the other hand, a second base table or frame 12 is made of soft iron material of electromagnetic property similarly to the first frame 4. A permanent magnet 13 and a pole piece 14 are respectively attached to this second frame 12 by a bonding agent. The other torquer coil 7' but being accommodated therein. This second frame 12 forms a magnetic circuit between the permanent magnet 13 and the pole piece 14 and hence serves as a yoke. Then, an engaging portion 16 of the second frame 12 is inserted into an engaging portion 15 of the first frame 4, whereby to fasten the first and second frames 4 and 12 integrally together.
The first frame 4 and the second frame 12 are respectively provided with stoppers 17 and 18 each of which limits the operation range of the pendulum 1. In this case, it is possible to freely adjust the limited operation range of the pendulum 1 by rotatably moving the stoppers 17 and 18, respectively by a screw driver.
The above-described component parts are already able to function as the accelerometer. These component parts are respectively fixed to a casing or housing 19 by a plurality of screws 20 are fastened integral with the housing 19. Then, a lid 21 serving as a characteristic name plate is bonded to the housing 19 to thereby make it possible to maintain the inside of the housing 19 air tight.
Outside terminals 22 such as a lead wire (not shown) and so on for deriving an electrical signal from the light receiving elements 10 and 10' or the like which form the displacement detector and for supplying a feedback current to the torquer coils 7 and 7' are respectively bonded to a terminal casing 23 in an air tight manner and the terminal casing 23 is bonded in an air tight manner to the housing 19 such that one ends of the outside terminals 22 are disposed within the housing 19. Through a central opening 24 of the terminal casing 23, the interconnection of an electrical system of apparatus disposed within the housing 19 and so on are carried out. Then, while evacuating the inside of the housing 19 to vacuum or while filling it with an inert gas (for example, helium gas) or the like, a lid 25 is bonded to the central opening 24 to thereby maintain the inside of the housing 19 air tight. Thus, the parts of the accelerometer therein can be prevented from deteriorating in quality and thence the efficiency of the accelerometer can be held stable for a long time of period.
In the accelerometer arranged as mentioned above, if acceleration acts in the direction of the central axis X--X of the housing 19, the pendulum 1 is displaced a little around the thin portion 2 of the hinge 3, and the light which is emitted from the light emission element 11 is shielded or divided by the pendulum 1 so that the light incident on the light receiving elements 10 and 10' of the displacement detector varies and thus an electrical differential signal corresponding to such displacement is derived from the elements 10 and 10'. Then, a current proportional to this electrical signal is fed back to the torquer coils 7 and 7', by which the pendulum 1 is restored so as to reduce the electrical signal outputs from the light receiving elements 10 and 10' to zero. Since this electrical signal, or the restricting current is proportional to the acceleration, it is possible to detect the input acceleration by measuring this restricting current.
In FIG. 2, reference numerals 19-3 designate apertures formed through a flange-shaped portion 19-4 of the housing 19 which is used to mount this accelerometer on an object whose acceleration is to be measured.
In the above-described accelerometer, however, the first frame 4 assembled to be operable as the accelerometer is accommodated in the cylindrical-shaped housing 19 such that a bottom surface 19-1 of the housing 19 is in contact with a lower surface 4-1 of the first frame 4 and then the first frame 4 and the housing 19 are fixed to each other with a plurality of screws 20. Thereafter, the whole of the accelerometer is mounted on the measured object with a mount reference plane 19-2 beneath the flange-shaped portion 19-4 of the cylindrical-shaped housing 19. Accordingly, even if the accelerometer elements accommodated within the first frame 4 are adjusted precisely, when the first frame 4 is assembled into the housing 19 such that there is incorrect parallelism or flatness between the lower surface 4-1 of the first frame 4 and the mount reference plane 19-2, this inaccurate parallelism or flatness becomes a bias error which varies a zero point. Further, the tightness of the plurality of screws 20 also causes the bias error and furthermore, and the bias error is easily affected by the temperature change.
FIG. 3A is an enlarged perspective view of the displacement detecting apparatus used in the prior art accelerometer shown in FIGS. 1 and 2.
Referring to FIG. 3A, a lamp holder 28 of columnar shape which holds the light emission element 11 is inserted into and then fixed within an opening 4-3 formed through the first frame 4 in FIGS. 1 and 2. Reference letter R--R designates the central axis of the lamp holder 28, while a central axis of an opening 28-1 provided through the lamp holder 28 into which the light emission element 11 is inserted or an optical axis S--S of the light emission element 11 is made parallel to the central axis R--R of the lamp holder 28 but displaced upward by .epsilon. in this example. Accordingly, if the lamp holder 28 accommodating therein the light emission element 11 is rotated around the R--R axis by using a slit-like groove 28-2 formed on the lamp holder 28, the optical axis S--S of the light emission element 11 can be slightly displaced in the X--X' direction in FIG. 3A, or in the moving direction of the pendulum 1 perpendicular to the axis of the pendulum 1.
Two light receiving elements 10 and 10' are respectively inserted into and fixed in openings 29-1 and 29-2 which are respectively formed through a columnar holder 29 and spaced apart from each other in the X--X' direction. Thereafter, the holder 29 is inserted into and then fixed to the opening 4-2 of the first frame 4. Accordingly, the light emission element 11 and the light receiving elements 10, 10' are opposed to each other across the free end portion serving as the light shielding portion of the pendulum 1. Reference numeral 29-3 designates a slit-like groove formed on the columnar holder 29 similar to the groove 28-2.
FIG. 3B illustrates the relationship among the light emission element 11, the light receiving elements 10, 10' and the pendulum 1. As shown in FIG. 3B, when the center O.sub.1 of the pendulum 1 is placed at the position coincident with a center axis O--O' which connects the optical axis of the light emission element 11 and the center of the optical axes of the light receiving elements 10 and 10', a bundle of light or light flux B emitted from the light emission element 11 is equally divided by the free end portion of the pendulum 1, which free end portion serves as the light shielding portion. As a result, substantially half of the surfaces which are smaller than the whole light receiving surfaces of the two light receiving elements 10 and 10' is equally irradiated with the divided light flux so that no voltage or zero voltage appears across differential output terminals A and A.sub.1 which are respectively led out from the light receiving elements 10 and 10'. In other words, FIG. 3B illustrates the state where the pendulum 1 is at its zero position.
However, when, for example, acceleration acts on the pendulum 1 to displace the same from the position shown in FIG. 3B in the X direction in the figure, the amount of light irradiating the light receiving element 10 becomes larger than that of the light incident on the light receiving element 10' so that a voltage corresponding to the displacement of the pendulum 1 is produced between the differential output terminals A and A.sub.1, and thus this apparatus functions as a displacement detecting apparatus.
Normally the zero position of the pendulum 1 is not always placed on the center axis O--O' of the light emission element 11 and the light receiving elements 10 and 10'. Further, under the condition where no acceleration acts on the pendulum 1, an output voltage is produced between the output terminals A and A.sub.1 due to the scattering of the characteristics of the light receiving elements 10 and 10'. Therefore, in the course of adjusting the displacement detecting apparatus, it becomes necessary to reduce the output voltage to zero.
In the prior art accelerometer shown in FIGS. 1 and 2, the lamp holder 28 can be rotated by a small angle as earlier noted, or as shown in FIG. 3C, the lamp holder 28 or the holder 29 can be rotated around the axis R--R in the direction shown by an arrow a to thereby move the light emission element 11 or the light receiving elements 10 and 10' in the right and left directions in correspondence with the zero position of the pendulum 1, and thus the zero position of the displacement detecting apparatus can be adjusted.
In such prior art accelerometer, however, since the movement of the free end portion of the rod-shaped pendulum 1 is detected by using the light emission element 11 and the light receiving elements 10 and 10' which are respectively spaced apart across the pendulum 1 in opposing relation to each other, due to the interposition of the free end portion of the rod-shaped pendulum 1, the distance between the light emission element 11 and the light receiving elements 10, 10' is inevitably made to be a considerable distance. There are thus defects in the prior art displacement detecting apparatus which has low resolution and sensitivity and such displacement detecting apparatus has a large amount of noise. Accordingly, when this prior art apparatus is operated as an accelerometer, it has low resolution and has a large amount of noise.